Apparatus and method for monitoring radiation effects at different intensities

ABSTRACT

A multi-channel electromagnetically transparent voltage probe transmission link system or method monitors a plurality of voltage signals at a plurality of test points of a device under test that is subject to a radiation field with at least two intensity levels. The system or method compares the monitored voltage signals at the first intensity level corresponding to the level at which the monitored voltage signals not being affected by the radiation field, and the second intensity level being greater than the first intensity level. The system or methods determines whether or not the device is affected by the radiation field by determining if the monitored voltage signals are affected by the radiation field at the second intensity level.

This is a divisional of application(s) Ser. No. 08/169,703 filed on Dec.17, 1993 and now issued as U.S. Pat. No. 5,440,227, which is adivisional of Ser. No. 07/862,621 filed on Apr. 2, 1992 and now issuedas U.S. Pat. No. 5,311,116.

FIELD OF THE INVENTION

This inventions relates to methods and apparatus for testing thesusceptibility of devices, such as circuitry, to electromagneticinterference (EMI).

BACKGROUND OF THE INVENTION

Analog and digital electronic circuitry and attendant wiring mayencounter serious operating difficulty in the presence of strongelectromagnetic radiation fields. Such radiation fields are generallyreferred to as Electromagnetic Interference (EMI) fields. The circuitsand attendant wiring may be shielded and filtered to provide someimmunity to large EMI fields. Methods and apparatus, therefore, arerequired to test the susceptibility of the circuits and attendant wiringto EMI fields.

EMI testing is typically performed in shielded enclosures known as"screen rooms" or faraday cages, which provide an electromagneticenvironment wherein only controlled EMI fields are present. ControlledEMI fields include, but are not limited to radiated near and far fields,stripline and TEM testing in the range of DC (more typically 10 KHz) to18 GHz.

Apparatus typically used inside the screen room includes current probesattached to a harness wire and a coaxial cable which sends the signalsdetected by the probes to a receiver outside the screen room, where theeffects of the EMI fields on the circuit are determined. Current probessuitable for monitoring current during EMI tests are commerciallyavailable. The Ailtech model number 91197-11 is one such device. Currentprobes, however, are not able to measure signals in the device undertest in many circumstances, for example, at trace conductors ofintegrated circuits or into open circuits. For such signals, voltageprobes are better suited.

To ensure the integrity of the screen room and the results of the EMItests, any voltage measuring apparatus within the screen room shouldminimally perturb the controlled EMI fields and should be energized by asignal from the device under test only. For example, any test apparatuswhich might reradiate EMI fields impinging on the device under test ormight otherwise inject any noise into the device under test must beavoided.

U.S. Pat. No. 4,939,446, which is assigned to the assignee of thisinvention, refers to one such voltage probe transmission link that istransparent to electromagnetic radiation fields for use in screen roomtesting. The transmission link uses a voltage probe, which includes acircuit grabber, such as a short insulated conducting clip, which isconnected to an electrically overdamped input conductor. The circuitgrabber is connected to the test point of the device under test. Theinsulation on the clip surface is coated, with any bright, metallicreflecting material, such as a silver paint or foil, to shield the clipfrom impinging EMI fields, thereby preventing the injection of signalsinto the device under test by the clip. The other end of the inputconductor is connected to a hybrid electrical/optical data transmitterhaving a high impedance input port, which also is located inside thescreen room. The transmitter has an optical output port that isconnected to a receiver by way of an EMI immune optical fiber. Thereceiver is located outside the screen room where the effects ofcontrolled EMI fields on the device under test are monitored, outside ofthe test electromagnetic radiation field.

The voltage probe input conductors may comprise a non-metallic threadcore that is impregnated with fine conducting particles and a rigid,non-metallic insulating sheath. The electrically overdamped inputconductors have a high distributed resistance so that they will not ringor tune at the frequencies of interest and, therefore, will not pick upenergy from the EMI fields. As a result, the voltage probe transmissionlink may be used to monitor voltages of a device under test in thepresence of a strong EMI field without effecting the device under testor the test results. The disclosure of U.S. Pat. No. 4,939,446 is herebyincorporated in its entirety herein by reference.

A commercial product, known by the tradename ETVL (ElectromagneticallyTransparent Voltage Monitor Link System), available from the assignee ofthis invention, Electronic Development Inc., is a commercial version ofthe voltage probe transmission link described in U.S. Pat. No.4,934,446. The ETVL product has a hybrid electrical/optical datatransmitter that has a single transmission channel that may have one ofthree signal formats, namely analog, digital, and pulse stretched. Onlyone signal format can be used at a time on the one transmission channel.

The commercial ETVL device and the device described in U.S. Pat. No.4,939,446 monitor only a single test point and one voltage waveform(single ended or double ended) of the device under test. They also useone voltage probe for providing a return current path from thetransmitter to the device under test.

SUMMARY OF THE INVENTION

It is, therefore, an object of the present invention to provide improvedvoltage probe transmission link apparatus and methods for monitoring theeffects of an EMI field on a device under test.

It is another object of the invention to provide a multi-channel,electromagnetically transparent, voltage probe transmission link systemthat can monitor simultaneously a plurality of voltage signal waveformsof a device or system under test. It is another object to provide formonitoring simultaneously the effect of radiation fields at a pluralityof locations without affecting the device under test or the testresults. It is another object to monitor a plurality of waveforms atdifferent locations along a circuit path. It is another object tomonitor a plurality of voltage waveforms of a device under test inside ascreen room to determine the effect of a controlled EMI field.

It is a further object of the present invention to provide amulti-channel voltage probe transmission link system that has arechargeable power supply. It is another object to provide a pluralityof voltage probe channels with a common (or shared) power supply toprovide for extended operation inside a screen room. It is anotherobject to provide each transmitter of a channel with a dedicated powersupply.

It is another object of the invention to provide a radiation hardened,low power electrical to optical data transmitter for use in a screenroom during extended periods of time.

It is another object of the present invention to provide a variableattenuator for attenuating the sensed voltage signals of the device thatare larger than the effective dynamic range of the electrical to opticaldata transmitter. It is yet another object to provide a voltage dividerattenuator that is transparent to electromagnetic radiation.

In accordance with this invention, a multi-channelelectromagnetically-transparent voltage probe transmission link systemfor sensing a plurality of voltage signals at a plurality of test pointsof a device under test subjected to a radiation field is provided. Oneaspect of the invention is directed to a multi-channel system comprisinga plurality of voltage probe transmission link channels, each channelincluding two voltage probes, each voltage probe comprising a circuitgrabber and an electrically overdamped input conductor in electricalcontact with the circuit grabber. The circuit grabber (or grabbers)contact the device under test to monitor the voltage signals at one testpoint and the input conductor (or conductors) electrically transmits thesensed voltage signals.

Each channel also includes an electrical to optical transmitter forconverting the voltage signals transmitted by the input conductor (orconductors) to an optical signal, and transmitting the optical signalover a suitable transmission line. The optical transmission line passesthe optical signal out of the radiation field to a receiver forreceiving the optical signal. The receiver processes the optical signaland provides a display signal corresponding to the sensed voltage signalat the test point. The display signal is then displayed and thewaveforms may be evaluated to determine the effect, if any, of theradiation field on the device under test at the one test point.

Each voltage probe transmission link channel is associated with one ofthe plurality of test points of the device under test. Because themulti-channel link system is used to identify changes in the monitoredvoltage signal waveforms caused by the radiation field, it is notnecessary that the displayed signals exactly display the sensedvoltages. Rather, the display signals need only reflect relative changesin the monitored voltage signal waveforms as a result of the radiationfield used during the test.

Each transmitter may be provided with a dedicated power supply such as arechargeable battery. Also, the plurality of transmitters may beconnected to a common (or shared) power supply by relatively shortshielded conductors. Further, both a dedicated power supply and a commonpower supply may be used. The common power supply may be one or morediscrete power supplies such that not all transmitters are connected tothe same power supply. If each transmitter also has a dedicated powersupply, the dedicated power supply may be switched out, automatically orby a switch, when the transmitter is connected to a common power supply.Preferably, the dedicated power supplies are located internal to thetransmitters.

The plurality of transmitters, including any internal or common powersupplies, are radiation hardened, either as an integrated system in anenclosure or as interconnected components, for the test radiation fieldintensities (V/m) and frequencies of interest.

Each transmitter is preferably releasably mountable on a common base orframe. The term releasably mountable means that each transmitter can besecured to and removed from a receptacle in the base and used to monitora voltage in both conditions. The transmitter can be secured in place byany means, e.g., pins, latches, keys, friction, bolts and nuts, etc.Securing each of the transmitters to a common base provides for easyportability of the equipment, for example, into, out of, and within ascreen room. It also provides for radiation hardening the base with theplurality of transmitters secured to the base.

In embodiments where a common power supply is used, the common powersupply may be built into the base, and the base, the common powersupply, and the plurality of cables connecting the common power supplyto the transmitters may be radiation hardened as an integrated assembledunit.

That each transmitter may be removed from the base provides for locatingeach transmitter proximate to the test point it is to monitor. This isadvantageous where the device under test is a large object, such as anautomotive vehicle or its electrical system or a local area network ofcomputers, and the distance between two transmitters for two test pointsbeing monitored is greater than the desired length for the inputconductor of the voltage probe. This in turn provides for using the samelength input conductor for each voltage probe, and maintaining thatlength to less than a meter. This is advantageous in view of distributedresistance of the input conductor material, which results in aresistance that is directly proportional to its length. As noted, eachtransmitter also may be advantageously provided with an internal powersupply. Thus, if the cable connecting the transmitter to the base powersupply becomes problematic with respect to radiation hardening, thecable may be omitted and the internal power supply switched in.

Removability of the converters also provides for using less than thefull plurality of transmission channels, rapid replacement of atransmitter that is in need of service (or a recharge when operating onan internal battery) and quickly and easily changing the mixture ofchannel types of the plurality of channels, as between analog anddigital transmitter channels. This is particularly advantageous whencomplicated digital integrated circuits having analog sensors is beingtested and an EMI susceptibility problem has been identified, yet needsto be better isolated along a signal path having analog and digitalsignals.

An advantage of a common power supply is that it may be larger and havea longer useful life than using a plurality of dedicated power supplieswhich preferably are small enough to fit into the transmitter enclosure.For example, the common power supply may be a heavier 4.8 amp-hourrechargeable battery having a battery life of 30 hours when connected totwo transmitters. In contrast, suitable internal power supplies for eachtransmitter may be lighter, smaller, and have a shorter useful life, forexample, a 12 volt 0.6 amp-hour battery having a useful life of sevenhours. Further, the internal battery may be automatically switched in ifthe common power supply becomes discharged below a threshold voltage ordisconnected, thereby extending the useful life of the voltage probetransmission link channels. Similarly, if any transmitter is not beingused, it may be automatically powered down or manually switched off sothat it does not unnecessarily drain the power supply.

Also, the common power supply (with or without the internal battery) maybe located adjacent the base, or outside of the radiation field andcoupled to the transmitters using appropriately shielded cables. In yetanother embodiment, the common power supply may be derived fromconventional line current that is converted to the DC voltage level usedby each system, preferably a regulated DC voltage.

Preferably, each of the transmitters is provided with a plug or areceptacle that is compatible with a corresponding receptacle or plug inthe base so that when the transmitter is secured to the base, it isconnectable to a common power supply. The connection may be automaticthrough the plug/receptacle connection, or it may be controlled by asuitable switch or conventional shielded cables and connectors.

Each of the plurality of voltage probe transmission links also comprisesa receiver, which is located out of the effective range of the testradiation field. Each receiver receives the optical signalscorresponding to the sensed voltage of one test point from the opticaltransmission line, and processes the optical signals to produce adisplay signal. The plurality of receivers are preferably releasablymountable on a common base and are respectively connected to a suitabledevice (or devices) for displaying the plurality of display signalscorresponding to the plurality of voltage signals monitored at thedifferent test points of the device under test. A suitable display maybe a multi-channel display device or a plurality of single channeldisplay devices, for example, one or more single or multi-channeloscilloscopes, spectrum analyzers, voltage meters, or similar devices.

Surprisingly, it was discovered that efforts to multiplex the opticalsignals corresponding to the plurality of test points, to permit use ofa single optical fiber passing out of the radiation field, tended tomask susceptible device voltage signal waveform changes that occurredduring RF testing. In particular, a prohibitively high sampling ratewould be required to detect small waveform changes in a waveform havinga 30 MHz frequency. Although multiplexing may be useful for a two orthree channel transmission link system at low radiation fieldfrequencies, such a system is not practical or sufficient to satisfy thecommercial needs of the users who require, for example, six (or more)channels to monitor a device under test at frequencies up to 18 GHz.

Another aspect of the present invention is directed towards anattenuator for attenuating a sensed voltage signal to a range suitablefor processing by a device having a limited input range, over afrequency range of interest. One embodiment of this aspect of theinvention concerns an attenuator for attenuating the sensed voltage atthe device under test for processing by a low power transmitter, whichhas a limited input signal range. One such attenuator includes a voltageprobe and a length of an electrically overdamped conductive wire havinga distributed impedance (resistance and capacitance) along its length,the length connecting the input conductor of the voltage probe to aground (virtual or actual). This results in the voltage input at theprobe being divided across the first length and the distance between thevoltage source and the location where the first length is connected tothe voltage probe input conductor. Thus, by adjusting the relativelocation of the connection along the probe input conductor, or byadjusting the length of the first length (or both), the magnitude of thevoltage source may be attenuated by a selectable amount. This providesfor a signal, corresponding to the monitored voltage signal, that has arelatively full scale peak to peak swing with respect to the transmitterinput capacity.

Preferably, the power supply (dedicated and/or common) is monitored by abattery charge monitor to provide an indication of the net charge on thepower supply. This is important because if the power supply voltagefalls below a preselected level, e.g., 10 volts, the transmittercircuits may not operate in a linear manner, and, if undetected, couldtransmit distorted signals that could be mistaken for signals affectedby a radiation field. The battery charge monitor could be used totrigger a switch to change automatically between a common power sourceand an internal power source such that the internal power source is usedas a backup power supply.

Another aspect of the present invention concerns another improvement toU.S. Pat. No. 4,939,446 concerning the electrical to optical transmitterand monitoring the voltage waveform at a test point of the device undertest. In this aspect, the input circuits to the differential amplifierof the transmitter are provided with a centertapped interconnection.This centertap provides a return current path for the signals of thedevice under test that are sensed by a voltage probe. Accordingly, theneed for a reference ground return voltage probe connecting the deviceunder test to the transmitter centertap voltage has been eliminated.Thus, no more than two voltage probes are now needed to monitor adifferential output voltage signal, in place of the three probespreviously required. For monitoring single ended output voltage signals,the second input circuit of the transmitter is preferably connected tothe ground of the device under test.

In a preferred embodiment, the centertap voltage return is obtained byrespectively inputting the sensed voltage signals from the two inputconductors into two potentiometers at the input circuits of thetransmitter, such that the other ends of the potentiometers areconnected to a common centertap voltage. Advantageously, this centertapreturn simplifies connection of each channel transmitter to a test pointof devices under test and reduces the number of voltage probes required.This reduces the cost of the multi-channel device and the time requiredto select and connect the voltage probes to the test point of the deviceunder test or reconnecting the voltage probes from one test point toanother. The savings are multiplied by the number of channels used.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features of the invention, its nature and various advantageswill be apparent from the accompanying drawings and the followingdetailed description of the invention in which like reference numeralsrefer to like elements and in which:

FIG. 1 is an isometric view of a test set-up for a multi-channel EMItransparent voltage waveform monitor link in accordance with a preferredembodiment of the present invention;

FIG. 2 is a block diagram of an alternate embodiment of the test set-upof FIG. 1;

FIG. 3 is a circuit block diagram for an analog channel voltage probetransmission link in accordance with an embodiment of the invention;

FIG. 3A is a circuit schematic for the analog channel electrical tooptical transmitter of FIG. 3;

FIG. 3B is a circuit schematic for the analog channel optical toelectrical receiver of FIG. 3;

FIG. 4 is the battery charge monitor circuit of FIG. 3A;

FIGS. 5A, 5B, 5C, and 5D are representations showing the effects of aradiation field for two test points of a device under test before andafter the effects appear;

FIG. 6 is a representation showing the effects of the radiation fieldfor the two test points of the device under test of FIG. 5 afterelectromagnetic susceptibility is corrected; and

FIG. 7 is a diagram of a voltage divider transparent to electromagneticradiation in accordance with a preferred embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a preferred embodiment of the multi-channel transmissionlink system of the present invention. Such a system includes a pluralityof channels, e.g., six or more, of which only four channels are shown.The four channels are respectively designated by the suffix letters a,b, c, and d. These suffixes are used throughout the specification todesignate corresponding elements of the same channel. As it will appearfrom the context of the discussion, the suffixes may be omitted when acharacteristic common to each of the channels is discussed.

The system shown in FIG. 1 has four transmitter modules 10a, 10b, 10cand 10d. Transmitters 10b, 10c, and 10d are shown mounted to a commonbase 20. Base 20 is illustrated in FIG. 1 as an L shaped rack, but mayhave any other convenient shape that is compatible with retainingtransmitters 10. Transmitter 10a is shown removed from base 20.Receptacles 21a and 23a on base 20 are visible. Receptacle 21 may beused for alignment and correct seating of transmitter 10. Receptacle 23may be used when coupling transmitter 10 to a common power supply (e.g.,supply 80 in FIG. 2).

Each transmitter 10 is used to monitor a different test point,respectively designated by reference characters a, b, c, and d, of adevice under test 100. Referring also to FIG. 3, each transmitter 10 hasa pair of voltage probes 301 and 302 such that a pair of circuitgrabbers 11 and 12 are respectively connected across the double endedoutput of the test point in device 100 for monitoring the voltage at thetest point. Each of the circuit grabbers 11 and 12 are respectivelyconnected to transmitter 10 by electrically overdamped conductors 14 and15. If the test point is a single ended output, voltage probe 302 neednot be used.

Regarding the circuit grabbers, they are metallic grabbers inserted atthe end of the overdamped conductors. The grabbers are coated with apaint that reflects E-fields up to 200 V/m over sweep frequencies ashigh as 18 GHz. A hard baked to coat of plastic, e.g., poly-urethane,such as Sherwin-Williams brand "Polane B", is applied over thereflective paint to prevent scratches that might destroy the reflectingcapacity of the paint. Untreated grabbers become more sensitive topickup at frequencies above 1.0 GHz.

To connect to an exposed wire or circuit lead, a clip type grabber maybe used. To connect to a harness cable coated with insulation, a pintype grabber that penetrates the insulation may be used. Thus, accordingto the present invention, a plurality of channels having respectivevoltage probe pins may be inserted at different lengths along a harnessto check for resonances. Other circuit grabber shapes and configurationsmay be used for securely fastening to the test point of the device. Thisincludes wires having a reflective coating soldered to test points. Thelatter is particularly useful if the device under test is being movedduring the test. Each circuit grabber may be one of the clip or pin asdescribed in U.S. Pat. No. 4,939,446.

Regarding the electrically overdamped input conductor, it is preferablya non-metallic material made from a glass and carbon slurry that iswrapped (or coextruded) with a rugged nylon protective sheath addconducts the monitored waveform therethrough by the well knowndisplacement current mechanism. These conductors are transparent toelectrical fields as high as 200 V/m in a frequency range between 10 KHzand 18 GHz, and to magnetic fields between 30 Hz and 80 KHz. Forexample, an acceptable probe input conductor is known by the tradenameFLUOROSINT® 719, available from the Polymer Corporation of Reading, Pa.It has a carbon/fluorocarbon core that is 0.030 inches (0.76 mm.) indiameter and enclosed in a transparent nylon insulating cover to yieldan outer diameter of 0.040 inches (1.02 mm.), and a resistance per unitlength of 20,000 to 30,000 ohms per inch (7874Ω/cm. to 11811Ω/cm). Theinput conductor material has a small amount of distributed capacitancethat causes sensed voltage signal waveforms, monitored from device 100,to roll off at high frequencies. This roll-off is compensated for byusing wide bandwidth amplifiers from DC to 40 MHz in the transmittercircuits. The input conductor may be a single strand or multiplestrands, e.g., 4 to 5 strands, connected in parallel to lower theeffective impedance, to increase its structural rigidity as a cable. Apreferred overdamped input conductor and circuit grabbers, all of whichare transparent to the radiation field, are further described in U.S.Pat. No. 4,939,446.

Referring to FIGS. 1-3, each transmitter 10 includes an electrical tooptical converter circuit 30. Converter circuit 30 has inputs 31 and 32for receiving the voltage drop across circuit grabbers 11 and 12, astransmitted by conductors 14 and 15, and power from a power supply, andhas as an output an optical signal.

The optical signal is coupled to an optical transmission line 50, e.g.,a conventional fiber optic cable (or cable bundle), that is transparentto the radiation field at the frequencies of testing. Transmission line50 has a length that is sufficient to extend from transmitter 10 out ofthe radiation field to which the device 100 is exposed, for example, 10meters or more. The power supply may be an internal battery 40 (see FIG.1), a common battery 80 that is shared by one or more other transmitters10 (see FIG. 2) or both (not shown).

In the embodiments shown in FIGS. I and 2, device under test 100includes a printed circuit board 110 having a microprocessor device 120and a receiver logic circuit 130 that are connected by a lead 140 and aseparate power supply 150 that is connected to circuit 130 by a harness160. Thus, test points a, b, c and d represent different locations alongone or more circuit paths of device 100.

The device 100, the transmitters 10a, 10b, 10c, and 10d, and base 20 arephysically located inside a conventional screen room 200. The screenroom 200 has a plurality of antenna devices 210 (four such antennas 210are shown) for providing a controlled EMI field (i.e., a controlledradiation field) for testing device 100 for the effects of the EMIfield, i.e., susceptibility to electromagnetic radiation,electromagnetic emission, or both. It is to be understood that theinvention is applicable to testing the effect of any radiation field onany device (or system) under test, whether or not the test is performedin a screen room, and whether or not the test is conducted using acontrolled radiation field, a near field emitted from a waveguide, astrip line, a TEM field, or an ambient (near or far) radiation field.

Referring to FIG. 1, the plurality of optical transmission lines, e.g.,lines 50a, 50b, 50c, and 50d, are passed through awaveguide-beyond-cutoff filter 220 located in the enclosure of thescreen room 200, to a like plurality of receivers 60, namely receivers60a, 60b, 60c and 60d. Filter 220 is used to prevent transmission ofelectromagnetic radiation in either direction through the aperture,which transmission may adversely affect the test.

Each of the receivers 60 is used to convert the optically transmittedsignal to a display signal, which can then be transmitted over acompatible conductor 68 for display on a display 70. Preferably, thedisplay signal is an electrical signal and conductor 68 is aconventional coaxial cable, such as RG-58/U type cable. Receivers 60 arepreferably removably mounted in a rack 65. Receivers 60 also may beprovided with dedicated 12 volt DC power supplies or a common 12 voltpower supply. These power supplies may be rechargeable batteriesinternal to each receiver or common to the receivers, or derived fromone or more AC to DC power supply devices operating from a wall outlet.Preferably, the receivers 60 are individually switched so that thereceivers 60 for unused channels may be turned off. Removability of thetransmitters 10 and receivers 60 also allows for replacing a unit inneed of service or in need of a battery charge with a fully operationaland charged unit, or changing a channel transmission link from a digitalto an analog channel or vice versa, without significantly delaying orinterfering with testing of the device under test.

Each channel of the multi-channel system may be an analog channel or adigital channel. Such types of channels are known and have been used inthe aforementioned commercial ETVL product, but only in a configurationthat has both channel types such that the user may select and use onlyone channel type at a time. Thus, in the present invention, eachtransmitter 10 is a single type of channel and both the number ofchannels and the type of channel may be selected to permit optimummonitoring of a wide variety of voltage signal waveforms simultaneouslyfrom device 100.

Analog channels are particularly useful for transmitting monitoredvoltage waveforms at frequency at and below about 5 MHz. Digitalchannels are particularly useful for transmitting frequencies above 5MHz, in which a fast rise and fall time are used, e.g., 25 nanosecondsor less. The digital channel provides a pulse stretching function,taking advantage of the distributed capacitance of the voltage probeinput conductor. The pulse stretching ratio is about a 4:1 ratio. Thisis particularly useful for viewing waveforms comprising narrow pulseperiods that are long with respect to pulse width.

As shown in FIG. 1, display 70 may be a multi-channel scope capable ofdisplaying the monitored signals from test points a, b, c, and d,simultaneously. Display 70 is preferably capable of monitoring thewaveforms of device 100 before and after the application of theradiation field of the test or, more specifically, the radiation fieldat two intensity levels. Typically, one radiation field intensity levelwill be selected because no effects of the radiation field appear in themonitored voltage signal waveforms and a second intensity level will beselected because the affect of the radiation field does appear in some,if not all, of the monitored test point signals.

Voltage waveform changes provide the user with both test failure anddiagnostic information during signal tracing. This information can beused to isolate the electromagnetic coupling or emission problem in thedevice 100, i.e., isolate the problem to a specific circuit component,element, or lead. Indeed, the present invention provides forsimultaneously monitoring different points along a signal path in acircuit or device and identifying the likely source of the EMI based onthe relative changes in the plurality of voltage signal waveforms alongthe signal path. Thus, the likely problem can be more quickly located.Further testing to isolate more specifically the problem can beconducted. Thereafter, suitable corrective steps can be implemented, forexample, insertion of a suitable RMI filter to resolve the EMIsusceptibility or emission problem in a given application of device 100.

Referring to the embodiment of FIG. 1, each transmitter 10 has adedicated power supply 40, which is preferably a 12 volt rechargeablelead acid battery. Preferably, the dedicated power supply 40 is locatedinternal to the enclosure of transmitter 10, which also enclosesconverter circuit 30 and the enclosure is radiation hardened. In such anembodiment, the battery harness connecting battery 40 to the power inputreference 325 of converter circuit 30 is located inside the hardenedenclosure and need not be separately shielded or electromagneticallytransparent.

FIG. 2 shows an alternate embodiment of a portion of the test set-up ofFIG. 1 wherein each transmitter 10' receives power from a common powersupply 80 over one of a plurality of conductors 82 In this embodiment,transmitter 10a', which is removed from base 20, is provided with powerfrom power supply 80 by an EMI shielded cable 82a. It is desirable tokeep cable 82a as short as possible.

Preferably, the common power supply 80 is associated with the base 20such that it is connected to the base 20 or enclosed interior to base20. This provides for enhanced radiation hardening of the base, commonpower supply, and the plurality of cables as an integrated unit. Forthose transmitters 10' that are removed from base 20 by a distancegreater than 15 meters, it may be desireable to provide suchtransmitters 10' with a dedicated, more preferably, internal, battery40. This avoids the risks of a long cable 82 coupling to the radiationfield or emitting radiation and affecting the test results. In suchcircumstances, use of an internal battery will optimize radiationtransparency.

A larger common power supply (not shown), located separate from the base20, or even outside of the radiation field also may be used withappropriate shielded cables. Preferably, the common power supply is oneor more rechargeable batteries located in the radiation field andradiation hardened. This is to avoid the problems inherent in using anAC to DC power supply in an electromagnetic, particularly an RF, fieldand using long battery harnesses.

Importantly, the multi-channel voltage probe transmission link system,and in particular the transmitters 10 (or 10') and their power supply,namely either a dedicated internal battery 40, a common battery 80, or aremote battery or power supply (not shown in FIGS. 1 and 2), is to beradiation hardened to at least the radiation intensity levels for thefrequencies of the test. This is so that the multi-channel system willnot radiate EMI during the test or couple to radiation from theradiation field, which could perturb the radiation field or injectsignals into the device under test and adversely affect the quality ofthe test.

Typically, each of transmitters 10 must be hardened against both nearand far fields to 200 volts per meter (V/m) over the frequency range offrom 10 KHz to 18 GHz. Base 20, together with any internal or externalcommon power supply 80 and cables 82, is similarly radiation hardenedwith the transmitters 10 secured to and removed from base 20. It also ispreferred that the test apparatus be compatible with the conventionalTEMPEST and EMP requirements.

Referring to FIGS. 3 and 3A, analog channel converter circuit 30 is alow-gain, direct-coupled, optical transmitter amplifier circuit whichtransmits analog waveforms through the transmitter output LED 310, intooptical cable 50, and to a compatible analog receiver circuit 60. LED310 is operated along a linear portion of its output characteristic sothat the intensity of the light emitted is directly related to themagnitude of the voltage drop at the test point.

Analog converter circuit 30 is connected to a test point of device 100(not shown in FIGS. 3, 3A) using two voltage probes 301 and 302 (notshown in FIG. 3A). Voltage probe 301 includes circuit grabber 11 and afirst length of overdamped input conductor 15 and voltage probe 302includes circuit grabber 12 and a first length of overdamped inputconductor 14. Preferably, the first length of input conductor 14 and thefirst length of input conductor 15 are the same. Regarding the testpoint signal of device 100, grabber 12 is connected to one of the "testpoint" and a "return path" in device 100, which may be a circuit groundof device 100, and grabber 11 is connected to the other of the testpoint and the return path, thereby to obtain a differential signalwaveform which is provided to inputs 31 and 32 of converter circuit 30.The differential waveform is input to a differential amplifier circuit320.

Referring to FIG. 3A and amplifier circuit 320, the signal input atinput 31 is passed across an input resistor 311, into one end of apotentiometer P1, and out the wiper contact of potentiometer P1 into thenoninverting input of amplifier A1. The input signal at input 32 issimilarly passed across an input resistor 312, a potentiometer P2, andthe noninverting input of an amplifier A2. Input resistors 311 and 312are each preferably 2.1 KΩ and are optionally used to prevent badoverdriving of converter circuit 30 in the event that potentiometers P1and P2 are not properly adjusted. Potentiometers P1 and P2 are gangedtogether to provide the same resistance. The ends of potentiometers P1and P2 on the other side of their wiper contacts from the input signalsare connected to a common centertap CT, and thereby provide a returncurrent path to the test point of device 100. Thus, the problem ofrequiring a separate voltage probe to provide such a return current pathto the device under test, as disclosed in U.S. Pat. No. 4,939,446, isovercome.

Potentiometers P1 and P2 each may be a 50KΩ potentiometer, although itis believed that other potentiometers having resistance values up to1.0MΩ may be used. In operation, potentiometers P1 and P2 are firstplaced in their full resistance positions, ganged together, and thenadjusted to reduce the resistance to a level that provides the desiredwaveform amplitude range into amplifiers A1 and A2.

Amplifiers A1 and A2 are preferably provided with the same voltage tocurrent converter amplifier configuration, i.e., a unity gain bufferamplifier having the output pin 6 fed back to inverting input pin 2. A+12 volt supply is provided at pin 7 of amplifiers A1 and A2 from the+12 volt battery, e.g., battery 40 (FIG. 1) or 80 (FIG. 2), obtained asillustrated from pin 3 of a commercial EMI filter 340 (and describedbelow). A virtual ground return is provided at pin 4 of amplifiers A1and A2, obtained from node VG, corresponding to the return path to the+12 volt supply and thus the point of lowest potential in circuit 30. Inthis embodiment, amplifiers A1 and A2 are insensitive to any relativevoltage drift in the power supply.

The outputs of amplifiers A1 and A2 are respectively fed to theinverting input at pin 2 and the noninverting input at pin 3 of adifferential amplifier A3. Amplifier A1 output at pin 6 is passed acrossa 1.8KΩ resistor 321 into pin 2 of amplifier A3. Amplifier A2 output atpin 6 is passed across a voltage divider circuit of potentiometer P3 andresistor 322 connected to the centertap CT (described below) into pin 3of amplifier A3. Potentiometer P3 is a 5KΩ potentiometer and resistor322 is a 1.8 KΩ resistor. They are used to provide a common moderejection for the selected input resistance to amplifiers A1 and A2. Inother words, with potentiometers P1 and P2 at their selected values andterminals 31 and 32 tied together, potentiometer P3 is adjusted toobtain a balance between inputs 31 and 32. Amplifier A3 is provided witha resistor 323 of 1.8KΩ in the feedback loop to provide unity gain.Other resistance values could be used, for example, to provide a lowgain other than unity.

The output of differential amplifier A3 at pin 6 is then input to theinverting input at pin 2 of amplifier A4. Amplifier A4 is configured asa unity gain inverting amplifier, having a 1.8KΩ resistor 324 at theinverting input and a 1.8KΩ resistor 326 in the inverting feedback loop.Amplifiers A3 and A4 are also provided with bias supplies of +12 volt atpin 7 and virtual ground at pin 4. The noninverting input of amplifierA4 is connected to the centertap CT.

The centertap CT is provided by a voltage regulator 345. Voltageregulator 345, suck as an MC7805, manufactured by Motorola, Inc.,converts the +12 volt supply and provides a regulated +6 volts outputacross a 0.1 μf decoupling capacitor 346 which is connected to virtualground at node VG. Thus, the +6 volt centertap CT is electricallyconnected to the noninput leads of potentiometers P1 and P2, to resistor322 and to the noninverting input of amplifier A4.

The output of amplifier A4 is passed to the base of transistor Q1 whichis a an RF transistor, such as model 2N3904 or the equivalent. Thecollector of transistor Q1 is connected to the +12 volt supply acrossdecoupling resistor 350 and capacitor 351 which are respectively 47Ω and0.1 μf. Capacitor 351 is tied to the virtual ground at node VG. Theemitter current of transistor Q1 is passed across a DC bias resistor 353of 390Ω and into light source 310. Light source 310 is preferably acommercial light emitting diode device, model No. HFBR 1404,manufactured by Hewlett Packard, having a nominal wavelength of 820 nm.

EMI Filter 340 is inserted between converter circuit 30 and the twoleads connecting the circuit to the power supply. It is used to suppresselectromagnetic susceptibility radiation over the frequency range ofinterest. It may be any filter suitable for such purpose, and preferablyis model BNX002, available from MURATA Manufacturing Co., Ltd.,Savannah, Ga., which has a flat filter response of between 0.01 and 1.0GHz.

In the design of converter circuit 30 of FIG. 3A, it is important thateach element connected to the virtual ground be directly connected tonode VG by a separate dedicated conductor (not shown), i.e., to theoutput pin 4 of EMI filter 340. This will minimize the circuit noise toa level that is at about 10 mV or less. The connection between the +6volt centertapped output CT of the +6 volt regulator 345, may, but neednot, be made by a separate conductor to each of potentiometers P1 andP2, resistor 322, and amplifier A4 (not shown).

The use of amplifier A4 as an inverting amplifier in converter circuit30 provides for simplifying the design of the compatible receiver 60.

Referring to FIGS. 3A and 4, the converter circuit 30 also includes abattery charge monitor 365, connected between terminals 3 and 4 of EMIfilter 340. Monitor 365 preferably comprises a bar graph driver device366, such as part no. LM3914N, manufactured by National Semiconductorand three different colored light emitting diodes (LEDs) 367, 368 and369. Device 366 is configured to turn on one of the LEDs when the powersupply is within the range of one of the LEDs. Thus, LED 367 ispreferably a red LED, e.g., Dialco part no. 558-0102-001, and isilluminated when the power supply is less than 11 volts; LED 368 ispreferably a yellow (amber or orange) LED, e.g., Dialco part no.558-0202-002, and is illuminated when the power supply is between 11 and12.8 volts; and LED 369 is preferably a green LED, e.g., Dialco part no.558-0302-001, and is illuminated when the power supply is between 12.8and 13.6 volts.

Monitor 365 is configured as shown in FIG. 4, with each of the LEDs 367,368 and 369 respectively connected in series between the power supplyand 5.1KΩ resistors and to pins 12, 11 and 10 of device 366. Regardingdevice 366, pin 3 is connected to the power supply; pins 1, 18, 17, 16,15, 14, 13 and 12 are tied together; pins 2, 4 and 8 are tied to thevirtual ground; pin 5 is connected to a bias voltage circuit including avoltage divider connected between the +12 volt power supply and virtualground, comprising a 100KΩ potentiometer P and 10KΩ resistor, forsetting the voltage thresholds for turning on and off the different LEDS367, 368 and 369; pins 6 and 7 are tied together; and pins 6 and 8 aretied together by a 1KΩ resistor. LEDs 367, 368 and 369 are preferablyvisible to the operator, more preferably conveniently located on orvisible through the enclosure of transmitter 10. Circuit 366 also may beconnected to a switch, for example, to actuate an audible alarm or topower down automatically the transmitter 10 when the voltage falls belowa threshold level, e.g., when the red LED is illuminated.

Converter circuit 30 thus has an automatic gain and bias adjust circuitthat maintains its output in the linear operating region of the lightsource LED 310. Amplifiers A1 and A2 provide unity gain between inputterminals 31 and 32 and the input of amplifier A3, and a low gain at theoutput of transistor Q1, which output drives LED 310. It is capable ofoperating with a power supply of between 10 and 15 volts. Below 10volts, there may be a loss of linearity in the circuit that could leadto inaccurate signal conversion and distorted waveforms. Accordingly,the threshold level of battery charge monitor is set somewhat above thevoltage where loss of linearity may occur. The circuit is a low powercircuit and requires only about 70 mA during operation. Accordingly, oneconverter circuit 30 can operate for about 15 hours on a 1.2 A-H rated+12 volt battery. The resultant DC drift is thus maintained in the mVrange and the noise level is on the order of 10 mV.

Referring to FIG. 3, an embodiment of an attenuator that may be used isshown. In this embodiment, a switch S1 is used to switch a short lengthof material R1 (i.e., the same non-metallic, electrically overdampedinput conductor of conductor 15) to connect input terminal 31 ofamplifier A1 to the virtual ground. Although shown interior totransmitter 10, the material R1 could be located outside of transmitter10.

The resistance of material R1, i.e., its length, is selected to be afraction of the length of conductor 15. Thus, when material R1 is placedacross the series resistance of probe 301, it reduces its input voltageby the fraction. A preferred fraction is one tenth. Thus, R1 is 1.9inches for a voltage probe input conductor 15 length of 19 inches. Whenactuated, switch S1 allows higher than normal TTL, CMOS and other commondevice 100 digital voltage waveforms to be monitored.

Referring to FIG. 7, another attenuator design is shown. In thisembodiment, a short length of material 17 (i.e., the same non-metallic,electrically overdamped conductor of conductor 15) connected between aselected location A on conductor 15 and the virtual ground VG. Theproportions of conductor 17 and the relative distance of point A fromcircuit grabber 15 are selected so that the combination of conductor 15and conductor 17 form a voltage divider. Thus, by adjusting position A,the point of electrical contact, the voltage divider value is selectedand then the voltage signal monitored at the test point can beattenuated to within the desired limits of converter circuit 30, e.g.,±6 volts, more preferably, ±3 volts. Such an attenuator also iselectromagnetically transparent and thus may be located internal orexternal to the transmitter 10 enclosure.

Thus, the present invention provides for using an attenuator to monitorsignals from device 100 that are as high as 150 volts, peak to peak, DCor AC, without driving amplifiers A1 and A2 into saturation, and withminimum distortion, i.e., below 10 mV of noise.

A compromise between low value resistors for maximum bandwidth andminimum current drain from the battery resulted in an amplifier circuitas illustrated in FIG. 3A that is capable of processing a sensed 5 voltsignal to produce a 1.0 volt output signal at 5 MHz, and to produce a0.5 volt output signal at 10 MHz has a maximum gain bandwidth product of1.0 MHz and a battery current drain on the order of 80 milliamperes.

Referring now to FIGS. 3 and 3B, a receiver 60 for receiving the opticalsignal transmitted from converter 30 of module 10 is shown. A lightdetector 380 is used to convert the analog optical signal to an analogvoltage signal. Detector 380 is preferably an integratedphotodiode-amplifier circuit, such as part no. HFBR 2404, available fromHewlett Packard. As shown in FIG. 3B, detector 380 has a variable DCoffset voltage, provided by circuit 390, that is used to compensate forchanges in the DC operating levels of the direct coupled amplifiers ofconverter circuit 30. Circuit 390 has a potentiometer P4 and a 0.1 μfcapacitor 391, connected in parallel to a virtual ground at node VG2(described below). Detector 380 has a regulated +5 volt supply, which isprovided by a +5 volt regulator 395, and which is passed acrossdecoupling resistor 396 and capacitors 397 and 398. Resistor 396 is 47Ω,and capacitors 397 and 398 are each 0.1 μf.

In the preferred embodiment, offset circuit 390 is used to overcome adiscovered limitation in design of the HP HFBR 2404 device, which has anAC coupled output that limits its DC output to a maximum voltage of+0.43 V. This voltage level is inadequate to drive RF transistor Q2,which is a model 2N3904 or equivalent transistor, to maintain a DC biasvoltage which is desirable for monitoring the effects of radiationfields.

As is noted below, one possible effect is an inversion of the monitoredvoltage signal. Thus, if the DC bias level of receiver 60 were 0, themagnitude of the inversion could not be monitored or evaluated. The sameis true for the output of receiver circuit 30. Circuit 390, however,provides for adjusting potentiometer P4 to raise the DC potential ofdevice HFBR 2404 to a level sufficient to drive transistor Q2, e.g.,between 0.5 and 1.5 volts. Potentiometer P4 could be replaced with afixed resistor when a desired bias is obtained. For example,potentiometer P4 could be set at or replaced with a resistor of 202Ω toobtain output of detector 380 at 0.75 volts. This provides for a biaslevel of +3 volts output at the collector of transistor Q3, which isadequate to display effects of the radiation field.

The output of the HP HFBR 2402 device is then passed to the base oftransistor Q2. The collector of transistor Q2 is provided with aregulated +6 volt supply from +6 volt regulator 392 that is passedacross a current limiting resistor 393 of 750Ω and a capacitor 394 (0.1μf). Capacitor 394 is connected to virtual ground VG2. Each ofregulators 392 and 395 are provided with a +12 volt supply at input 410(across on/off switch 52 and fuse F) and a virtual ground at node VG2.The virtual ground VG2 is at input 420 and is the return current path tothe +12 volt DC power supply for receiver 60. As is the case withconductor circuit 30, every element of circuit that is connected to thevirtual ground is shown connected to node VG2 by a separate conductor tominimize noise.

The emitter of transistor Q2 is connected to virtual ground VG2 across abias level circuit including potentiometer P5 (1.0 KΩ potentiometer) inparallel with a capacitor 399 (0.01 μf). Potentiometer P5 is adjusted toset the proper bias level for transistor Q2 to have the quiescentoperating point centered on the load line of the transistor.

The collector of transistor Q2 is connected to the base of transistorQ3, which is a 2N3904 transistor or equivalent. The collector oftransistor Q3 is connected to the +12 volt supply over decouplingresistors 401, each of which is 47Ω, and capacitors 402, each of whichis 0.1 μf, as illustrated in FIG. 3B. The emitter of transistor Q3 isconnected to virtual ground at node VG2 across resistor 403, a 330Ωresistor, and to output V_(out) across a resistor 404, a 50Ω resistor.Other decoupling resistors 401 (47Ω) and capacitors 402 (0.1 μf) areillustrated in FIG. 3B.

The +6 volt regulator 392 and +5 volt regulator 395 are used to providedrift control for circuit 60, to maintain DC drift-to less than 10 mV.The receiver circuit 60 provides output signals that vary by about 3-4volts in response to the optical input signal. Thus, it is preferred touse a display device 70 that has an adjustable gain, such as anoscilloscope or multi-channel oscilloscope, to amplify the output signalat V_(out) to a desired amplitude peak to peak swing range, e.g., ±6volts.

Circuit 60 also includes a circuit for adjusting the bias level of thesignal V_(out) by incorporating a potentiometer P6 across the inputterminals 410 and 420 and using the potentiometer wiper as a referenceoutput REF. Thus, by adjusting the potentiometer P6, the DC bias levelof signal V_(out) may be selected without affecting the waveform ofsignal V_(out). Potentiometer P6 preferably is a 3KΩ potentiometer.

Receiver circuit 60 is preferably provided with a 50 ohm output whichmay be varied as needed to be coupled to a display device.

The DC offset voltage shift circuit 390 for device HP HFBR 2404 is notrequired for optical transmission links that do not transmit DCwaveforms or that employ an analog-to-digital converter at the output ofthe optical receiver device chip. AC coupling for digital signals at arate greater than 0.1 Hz may be used. The light source LED 310 and lightdetecting photodiode-amplifier 395 in such instance, carry digital datawhich is not affected by the nonlinear-LED transmission-characteristics.The problem with such an approach, however, is that it masks changes inthe sensed voltage waveforms that occur when they become susceptible toEMI. Digital logic circuits tend to ignore device under test waveformchanges that indicate the onset of EMI susceptibility until the changesbecomes sufficiently large to be catastrophic. Thus, digital logiccircuits do not readily identify the onset of EMI susceptibility.

A transmitter and a compatible receiver for a digital channel voltageprobe transmission link may be adapted by a person of ordinary skill inthe art from FIGS. 3, 3A and 3B, and by referring to U.S. Pat. No.4,937,446, specifically to FIG. 3 and column 4, line 39 to column 5,line 3 of that patent, and the digital channel of the aforementionedETVL commercial product. The digital channel converter and receivercircuit architecture should be essentially the same as the analogtransmitter 10 and receiver 60 configuration indicated in FIGS. 3, 3Aand 3B. However, transmitter amplifier A4 is not required for a digitalchannel and may be omitted or replaced with a transistor amplifier. Withrespect to the digital receiver, a digital to analog converter may beused to reproduce analog signal waveforms at V_(out).

The 25 nanosecond rise and fall times of the digital transmission linkswitching waveforms are governed by the distributed capacitance of thevoltage probe input conductor nonmetallic material. Digital channel risetimes are 30 nanoseconds. The key to a high gain bandwidth product isthe ability to maintain distributed capacitance as low as possible. Acompromise in amplifier chip and discrete resistor value vs. currentdrain is obtained. This resulted in a 80 milliampere current drain and abandwidth from 0.1 Hz to 30 MHz. The digital receiver also is configuredto provide a 50 ohm output impedance in order to accommodate spectrumanalyzer monitoring of monitored waveforms from device 100.

The HFBR 1404 transmitting LED device and the HFBR 2404 receiver LEDdevice are well matched for use in the present invention.

A Hewlett Packard 8012B pulse generator having rise and fall times asshort as 5 nanoseconds may be used as a design tool. The digitaltransmission link should follow these rise and fall times well enough toprovide a 4-volt signal output with a 4-volt generator input at 10 MHz.Slight changes in the generator rise and fall times and height should beeasily detectable at the digital channel output.

The hardened optical receiver circuit 60 is a direct coupled two stageRF amplifier composed of discrete circuits for maximum switching risetime and fall times. This is not necessary when in the presence of farfields generated by antennas that are one meter, or further, from thedevice 100 and optical transmission link. Grounding the transmitter 10and base 20 prevents near field (less than 3 meters) capacitive couplinginto the transmitter 10 enclosures and interfering with the internalcircuitry. Capacitively coupled RF will penetrate any metal enclosureregardless of how thick, unless it is connected to a good low impedanceRF ground.

The effect of susceptibility to electromagnetic radiation of a deviceunder test is seen referring to FIGS. 5A, 5B, 5C and 5D and using atwo-channel transmission link. These figures reflect data recorded in ascreen room enclosure at different intensity levels of a controlled EMIfield and frequencies using a digital channel.

FIGS. 5A-5D are based on photographs of an oscilloscope displayingsignals monitored by a two channel voltage probe transmission link ofthe present invention wherein one voltage probe included a clip typegrabber monitoring a 5 volt DC input pin on a transmitter microprocessorand the other voltage probe included a clip type grabber monitoring theoutput pin of a logic gate in the receiver module of device 100. Themonitored transmitted pulse train in FIGS. 5A-5D is on the bottom trace.The monitored received pulse train is on the top trace. The reduced risetime of the received pulse is due to the effect of harness capacitance.

The waveforms just before susceptibility at 50.68 MHz and a fieldintensity level of 45 V/m are shown in FIG. 5A. FIG. 5B illustrates theeffect of raising the field intensity level to 78 V/m and maintainingthe frequency constant. A comparison of FIGS. 5A and 5B indicates thatthe controlled radiation field affected the signal transmission withinthe device under test. The transmitted pulse contains modulated RF andthe received pulse is widened and inverted.

FIG. 5C shows the waveforms at 101.8 MHz and below 20 V/m, just beforethe onset of susceptibility. It can be seen that the transmitted pulsesare modulating the RF at this point, but not enough noise is present toaffect the received pulses. FIG. 5D shows the increase in susceptibilitythat occurred with just a slight increase in field intensity level, from20 V/m to 35 V/m, at that same frequency.

The information illustrated in FIGS. 5A-5D could not have been obtainedby visually monitoring the voltage displays of the device under test orwith conventional current probes used in susceptibility testing or asingle channel commercial ETVL product.

The 5-volt DC input pin on the device microprocessor transmitter wasdecoupled to ground with a suitable EMI filter, and another EMI filterwas placed in series with the microprocessor output lead to an inputsignal conditioning circuit in the receiver of the device. The deviceunder test was thus made EMI compatible with the above filters.Referring to FIG. 6, which is in the same format as FIGS. 5A-5D, plotsrepresentative of a frequency sweep from 20 to over 200 MHz with the Efield intensity level at 100 V/m are shown. These signals indicate thatthe waveform distortion from the radiation field had been essentiallyeliminated. A minor (1%) increase in the transmitted pulse rate wasnoted between 191 and 204 MHz.

Referring to FIG. 1, transmitter 10 includes a rechargeable 12-voltbattery 40 which can be switched out when an external battery 80 (FIG.2) is used. The internal battery is preferably a 1.2 AH type thatprovides 10 hours of operation for a digital channel module and 15 hoursof operation for an analog channel module before recharge is required.

Referring to FIG. 2, an external battery module 80, coupled totransmitter 10 with TWINAX cables also can be used to provide power forthe plurality of transmitters 10 when extended periods of use betweenrecharge are required. TWINAX cables are available from Belden Wire andCable. Preferably, a +12 volt 4.8 amp hour rechargeable battery packlocated within the radiation hardened base 20 is used to provide powerto six transmitters 10. Suitable shielded leads and EMI filters may besupplied in series with base 20 and the input power terminals of eachtransmitter 10 to reduce the possibility of RF (EMI) interference. Alarger 60 AH battery may be used external to base 20 with suitableshielded cables and radiation hardening precautions.

Each receiver 60 is-preferably powered with a 1.2 amp, 24-volt DC U.S.or European compatible wall outlet supply. The supply can provide powerfor up to 6 receiver modules.

Each of the optical channel converter circuits 30 and receivers 60 arepreferably enclosed in 5 inch high, by 1.5 inch-wide, by six inch deep,EMI hardened circuit, modular metal cases. Internal module circuitry iselectrically and physically isolated from the metal enclosure. Isolatedprinted circuit boards with ground planes prevent RF currents on theenclosure exterior from entering the transmitter interior or circuitry.Particular attention is paid to package seams in order to preventinterior enclosure resonances at RF frequencies having wavelengths thatapproach the seam dimensions.

EMI power filters 340 are also provided within each transmitter module10 (see FIG. 3), as an additional measure, to prevent RF from enteringthe transmitter circuitry through the shielded power leads. BNCconnectors that mate the nonmetallic test probes with the transmitterprovide additional EMI immunity.

Screws located on both the individual optical transmitter modules andmodule rack allow them to be grounded to a copper table top or other RFground point when in the presence of near fields generated by strip linefixtures or TEM cells. Shielded power leads and EMI filters at the powerinput of each transmitter module may be required in order to providemaximum transmitter immunity to RF fields as high as 200 V/m between 10kHz and 18 GHz.

It should be understood that the transmitter configurations that aredisclosed in U.S. Pat. No. 4,939,446, which uses three probe connectorsto monitor a differential voltage at one test point, and a configurationthat uses only two probe connectors for monitoring single ended outputsof device 100, also may be used in the multi-channel embodiment of thepresent invention.

One skilled in the art will appreciate that the present invention can bepracticed by other than the described embodiments which are presentedfor purposes of illustration and not of limitation.

I claim:
 1. A method of monitoring the possible effect of a radiationfield on a device under test comprising:(a) monitoring simultaneouslyvoltage signals at a plurality of test points in the device; (b)subjecting the device to a radiation field having at least a first and asecond intensity levels; (c) comparing the monitored voltage signals atthe first intensity level and the monitored voltage signals at thesecond intensity level, the first intensity level corresponding to thelevel at which the monitored voltage signals are not being affected bythe radiation field, the second intensity level being greater than thefirst intensity level; and (d) determining from the comparing stepwhether or not the device is affected by the radiation field in responseto any of the monitored voltage signals being affected by the radiationfield at the second intensity level.
 2. The method of claim 1 whereinthe plurality of test points correspond to different locations along acircuit path in the device under test, further comprising:(e)determining if the device is affected by the radiation field at thesecond intensity level; and (f) identifying which of the locations alongthe circuit path is most affected by the radiation field at the secondintensity level based on the relative changes of the monitored voltagesignals at the plurality of locations at the first and second intensitylevels.
 3. The method of claim 2 further comprising:(g) selecting newtest points to be monitored in response to identifying one or more testpoints as most affected by the second intensity level and repeatingsteps (c) and (f) to isolate the test point most affected by theradiation field at the second intensity level.
 4. The method of claim 3wherein step (g) further comprises repeating steps (c) and (f) atdifferent intensity levels.
 5. An apparatus for monitoring the possibleeffect of a radiation field on a device under test comprising:means formonitoring simultaneously voltage signals at a plurality of test pointsin the device; means for subjecting the device to a radiation fieldhaving at least a first and a second intensity level; means forcomparing the monitored voltage signals at the first intensity level andthe monitored voltage signals at the second intensity level, the firstintensity level corresponding to the level at which the monitoredvoltage signals are not being affected by the radiation field, thesecond intensity level being greater than the first intensity level; andmeans for determining whether or not the device is affected by theradiation field in response to any of the monitored voltage signalsbeing affected by the radiation field at the second intensity level. 6.The apparatus of claim 5 wherein the plurality of test points correspondto different locations along a circuit path in the device under test,further comprising:means for determining if the device is affected bythe radiation field at the second intensity level; and means foridentifying which of the locations along the circuit path is mostaffected by the radiation field at the second intensity level based onthe relative changes of the monitored voltage signals at the pluralityof locations at the first and second intensity levels.
 7. The apparatusof claim 6 further comprising:means for isolating the test point mostaffected by the radiation field at the second intensity level.